1. Field of the Invention
This invention relates generally to telecommunications, and more particularly, to wireless communications.
2. Description of the Related Art
Service providers are constantly exploring various ways to generate more revenue while meeting demands of customers in different network environments including Intranet, Extranet, and e-commerce applications. For instance, telecommunication service providers exchange fee-based wireless and wireline traffic between mobile users and communication nodes, such as access points (APs) over a network to provide a variety of services to residential and business customers. An access point may be a transceiver that connects devices on a wireless local area network (WLAN) to the wired infrastructure. While an access point may be used by service providers to assure end-to-end quality of service and bandwidth guarantees over different network environments, a telecommunication service provider may offer Internet Protocol (IP) telephony and other network enhanced communication services to these customers. In doing so, these providers may employ optical and wireless networks, Internet infrastructure, communications software to enable, for example, Web-based enterprise solutions that link private and public networks.
One well-known standard, i.e., the Institute of Electrical and Electronics Engineers (IEEE) 802.11 specification describes the operation of mobile stations (MSs) and access points in a Wireless Local Area Network (WLAN). For a layered communication network protocol, this specification identifies both the physical layer (PHY), which details the nature of the transmitted signals, as well as the medium access control (MAC) layer, which defines a complete management protocol for interaction between mobile stations and access points. For more detailed discussion on the IEEE 802.11 standard (std.), one may refer to “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications,” published as IEEE std. 802.11, in 1999.
Specifically, at least three versions of the IEEE 802.11 standard exist, all sharing the same MAC 802.11b layer which operates in the 2.4 Giga Hertz (GHz) frequency band and has a PHY layer based on code division multiple access (CDMA), offering a peak data rate of 11 Mega bit per second (Mbits/s). The 802.11a and 802.11g versions operate in the 5.2 and 2.4 GHz bands respectively, both sharing a PHY layer based on orthogonal frequency division multiplexing (OFDM), offering a peak data rate of 54 Mbits/s. The IEEE 802.11 specification allows interoperability between wireless communication equipment from multiple vendors, and is commercially marketed as “Wi-Fi.”
Space Division Multiple Access (SDMA) has been studied extensively over the past few decades as a tool that uses spatial dimension to simultaneously transmit to, or receive from, multiple radios at the same carrier frequency. For more detailed discussion on the use of the spatial dimension to allow discrimination among multiple radio, one may refer to A. T. Alastalo, M. Kahola, “Smart-antenna operation for indoor wireless local-area networks using OFDM”, IEEE Transactions on Wireless Communications, vol. 2, no. 2, pp. 392-399, March 2003 and P. Vandenameele, L. Van Der Perre, M. G. E. Engels, B. Gyselinckx, H. J. De Man, “A combined OFDM/SDMA approach”, IEEE Journal on Select Areas of Communications, vol. 18, no. 11 pp. 2312-2321, November 2000.
However, the application of SDMA to wireless mobile communication systems, especially to cellular systems, such as Global System of Mobile Communications (GSM), cdma2000 and Universal Mobile telecommunication Systems (UMTS) has not always been successful. While simple implementations in the form of a fixed sectorization have been found to be effective, more sophisticated schemes, such as dynamic beam-forming, have been difficult to implement due to serious incompatibilities with the multiple access protocols in the above-cited cellular systems. Therefore, the application of sophisticated techniques for increasing the data rates available to mobile stations on a downlink that both may comply with the IEEE 802.11a/g standard specifications has not been adequately addressed in the literature for many reasons.
One reason for a lack of a high throughput downlink is that in most wireless LANs, the radio conditions are different at a transmitter and a receiver. As shown, FIG. 3 illustrates a stylized representation of a transmission protocol defined at least in part by IEEE 802.11 standard between a transmitter and a receiver where the transmitter transmits a MAC protocol data unit (MPDU) following listening and backoff, and in turn, the receiver transmits an acknowledgment (ACK) frame subject to a successful reception of the MPDU. The transmitter has no way of knowing whether the transmitted data was received correctly at the receiver. To this end, the IEEE 802.11 specifications state that upon a successful reception of a data burst (i.e., an MPDU), the receiver should send an acknowledgment frame (ACK) to the transmitter as confirmation. Should the transmitter not receive an ACK frame, it will assume a lost MPDU and will attempt re-transmission. The time interval between the last symbols of the MPDU and the first symbol of the ACK frame is referred to as a Short Inter-frame Space (SIFS) interval and is fixed at 16 μs in IEEE 802.11 networks. While the duration of a MPDU is arbitrary, the duration of an ACK frame is between 24 and 44 μs, depending upon the modulation and coding PHY parameters.
More specifically, the IEEE 802.11 standard MAC protocol is based on carrier-sense multiple-access with collision-avoidance (CSMA/CA). This MAC protocol essentially describes a “listen before you talk” access mechanism, whereby a IEEE 802.11 radio (mobile or access point) listens to the communication medium before starting a transmission. If the communication medium is already carrying a transmission (i.e., the measured background signal level is above a specified threshold), the radio will not begin its transmission. In such circumstances, the radio enters a deferral mode, where it has to wait for a period over which the medium is idle before attempting to transmit. This period is the sum of a Deterministic Inter-frame Space (DIFS) interval (34 μs in 802.11a and g) and a stochastic backoff interval (a re-transmission delay) with discrete values uniformly distributed over a range. The value of this range doubles with every unacknowledged transmission, until a maximum limit is reached. Once a transmission is successfully received and acknowledged, the range is reduced to its minimum value for the next transmission.
Providing increased downlink throughputs to legacy IEEE 802.11 mobile stations is an important distinguishing feature and marketing tool. However, multiple acknowledgement (ACK) bursts from different mobile stations may cause a reception problem upon their arrival at an access point. Likewise, accurate channel estimations may severely impact on successfully increasing the downlink throughputs. Therefore, without requiring a modification to the legacy IEEE 802.11 compliant mobile stations, a substantial increase in data rates using a single carrier frequency is not readily apparent on a downlink from an access point to the mobile stations in a WLAN.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.